app: reopening a chat after a CLI turn starts a spurious "app closed unexpectedly" resume turn #2474

Seen on macOS 15.6 while testing the #2472 idle sweep (since reverted in #2473). Engine: app/v3 2c717c0 (the #2472 merge commit), standalone codeaf app driven over its wire.

Steps:

1. The app engine opens 5 chats. The idle sweep releases the 2 oldest after 10 minutes.
2. codeaf chat --once (CLI, same home) runs a turn in the oldest, released chat. It exits 0 after 7.5 s.
3. The app engine sends a message into that chat. That works (turn.done ok:true).

Unexpected: after reopening the chat, the app engine started a turn of its own, beginning "The app closed unexpectedly while you were in the middle of answering…". That costs a model call and adds an entry nobody asked for.

Likely: the app treats the CLI's finished turn (or its leftover markers) as an interrupted turn of its own and resumes it. This may also happen without #2472, any time the app reopens a background chat that another process ran a turn in. That hasn't been verified on pre-#2472 code.

Logs from the run: fds/live2.log and fds/live2/A.wire.jsonl in the push's working folder on the Mac.

Pre-existing on app/v3 without #2472: yes.

Binary: codeaf built from 95ead2951, which is app/v3 3a6aa31 + #2466 with no #2472; sha256 prefix 6b10751e4f34f0f4. This is the soak slot's engine. Run as a standalone codeaf app on a scratch home, driven over its wire:

1. The app engine creates a chat with one message and never opens it as a view. Its first turn ends ok:true.
2. The idle sweep releases it after 10 minutes, so its journal is no longer open in the app process.
3. codeaf chat --once --session <its transcript> runs one turn in it from the CLI on the same home. The prompt asks for a shell command; with no approver attached the tool is blocked and the turn answers that. It exits 0 after 4.0 s.
4. The app engine calls agents.open on the chat, with no send. About 1 s later it starts turn 2 on its own. The journal gets a user line "The app closed unexpectedly while you were in the middle of answering the person…" and then a "[carry on] codeaf's completion check read a clipped summary of this turn…" line. The turn runs for about 18 s of model calls and ends ok:true.
5. A send afterwards runs normally as turn 3.

So reopening any background chat that another process ran a turn in starts this resume. An open is enough; no send is needed. #2472 only made this reachable for chats that were also open in the window. Logs: fds/resume1.log and fds/resume1/A.wire.jsonl in the push's working folder on the Mac.

Untested and worth checking first: the same path may trigger when a chat ran a turn on another computer (relay / "Continue here") and is then opened here. That would hit every relay user.

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